Secondary alcohol additives for lithium bromide-water absorption refrigeration system

ABSTRACT

A refrigerant-absorbent solution as a working fluid for use in absorption refrigeration systems that produces a substantial increase in the overall capacity of the refrigeration machine by improving the heat transfer in the absorber, by promoting dropwise condensation of the refrigerant vapor on the exterior surfaces of the condenser tubes and by improving purging of relatively noncondensible gases of the absorption machine. The working fluid comprises water as the refrigerant in combination with lithium bromide as the absorbent to which is added additives such as 4-heptanol, decyl alcohol, 2-phenoxyethanol, nonanol, 3methyl-2-heptanone, methyl octanoate, 3-methyl-2-heptanol, 1heptanol, 1-octanol, 2-hexanol, 2-octanol, 2-methyl-2-hexanol, 2methyl-2-heptanol, 6-methyl-2-heptanol, and 2-heptanol. These additives are supplied to the lithium bromide-water fluid in the range of 50 to 1,000 parts per million on a weight basis of the lithium bromide-water solution.

United States Patent [72] lnventors Chang W. Chi

Chicago; Robert A. Macriss, Deerfield; William F. Rush, ArlingtonHeights, all of Ill.

[21] Appl. No. 702,294

[22] Filed Feb. 1,1968

[45] Patented Sept.28,l97l [73] Assignee The American Gas Association,Inc.

[54] SECONDARY ALCOHOL ADDITIVES FOR LITHIUM BROMIDE-WATER ABSORPTIONOTHER REFERENCES Richtor, G. H., Textbook of Organic Chemistry, Wilsy,New York, 1943, pp. 72 and 78 ABSTRACT: A refrigerant-absorbent solutionas a working fluid for use in absorption refrigeration systems thatproduces a substantial increase in the overall capacity of therefrigeration machine by improving the heat transfer in the absorber, bypromoting dropwise condensation of the refrigerant vapor on the exteriorsurfaces of the condenser tubes and by improving purging of relativelynoncondensible gases of the absorption machine. The working fluidcomprises water as the refrigerant in combination with lithium bromideas the absorbent to which is added additives such as 4-heptanol, decylalcohol, Z-phenoxyethanol, nonanol, 3-methyl-2-heptanone, methyloctanoate, 3-methyl-2-heptanol, l-heptanol, l-octanol, 2-hexanol,2-octanol, 2-methyl-2-hexanol, 2-methyl-2- heptanol,6-methyl-2-heptanol, and Z-heptanol. These addi tives are supplied tothe lithium bromide-water fluid in the range of 50 to 1,000 parts permillion on a weight basis of the lithium bromide-water solution.

PRESSURE SOLUTION WITH Z-HEPTANOL ga /v20 PATENTEUSEP28I9?! 3609087,

F i l SCHEMA T/C DIAGRAM 0F STAT/C ABSORPTION APPARATUS VALVE A VALVE BPRESSURE GAGE mwmrm REA Dow HEAT/N6 {fl RECORDER com TRAP c040 BATHTEMPERATURE 5' Yfl52f fl( !l T f F i 2 [u COMPARATIVE PRESSURE 0;HISTORY a 1 5, 0 I W/TH NO/E ADD/77 & C

- ga -H 0 SOLUTION WITH 2-HEP7'ANOL D INVIiN'l'U/(S CHANG m cm, ROBERTA. MAOQ/SS E W/LL/AM FT RUSH b pan Fhwnum M0maab TIME /4 TTORNEYSSECONDARY ALCOHOL ADDITIVES FOR LITHIUM BROMIDE-WATER ABSORPTIONREFRIGERATION SYSTEM FIELD This invention relates to refrigeration inaccordance with the absorption refrigeration principle and moreparticularly to an absorbent possessing improved characteristics overknown working fluids for use with absorption refrigeration systems.

BACKGROUND Generally speaking, an absorption cycle uses two fluidstreams in a totally enclosed system. One of these fluid streams is therefrigerant, which provides the cooling effect; the other is theabsorbent, which conveys the refrigerant through part of the cycle. Themajor components of the system are a generator, condenser, evaporator,absorber, and heat exchanger. The refrigerant passes through all units;the absorbent is confined to movement through the generator, heatexchanger, and absorber. In this cycle no mechanical compressor isneeded.

In operation, a mixture of absorbent and refrigerant is heated in thegenerator to boil off some of the refrigerant, which rises as vapor tothe condenser where is it condensed as a liquid. The generator andcondenser operate at relatively high pressure, so the condensingtemperature of the refrigerant is sufficiently high to permit rejectingthe latent heat to the ambient air or cooling water. The liquidrefrigerant is throttled to lower pressure so it will boil at relativelylow temperature in the evaporator to which it is conveyed and thusabsorb heat from the air to be cooled while vaporizing. The vaporizedrefrigerant passes to the absorber, where it dissolves in cool absorbentsolution which has come to the absorber from the generator outlet. Thecool solution, now rich in refrigerant, is pumped back to the generatorto continue the process. In the past, a lithium bromide-watercombination refrigerant-absorbent has been utilized in systems of thetype described above. An absorbent solution with a greater rate ofrefrigerant absorption will increase the capacity and/or efficiency ofsuch machines.

OBJECTS It is therefore an object of this invention to provide anabsorbent solution that has an increased rate of refrigerant absorptionas compared to prior art working fluids.

It is another object of the present invention to provide a substantiallybetter absorbent solution or working fluid for use with absorptionrefrigeration systems than those which have been used in the past.

A further object of this invention is to provide a refrigerantabsorbentsolution for use primarily in the region of the absorber and condensertubes of the absorption machine which gives a general increase inrefrigeration capacity.

Still another and more speciflc object of this invention is to provideadditives which when added to lithium bromide-water working fluidaffords an increase in the absorption of heat of the refrigerantabsorbent solution.

Another object of this invention is to provide an improved additive forlithium bromide-water absorption refrigeration systems which assists increating a heat transferring turbulent film on the exterior of theabsorber tubes, provides improved purging of the relativelynoncondensible gases involved in the absorption machines, and permitsimproved dropwise condensation of the refrigerant vapor on the exteriorsurfaces of the condenser tube thereby resulting in an improvedcondenser heat transfer.

Still other objects will be evident from the detailed description whichfollows:

DETAILED DESCRIPTION Details of the invention will be described withreference to the following drawings:

FIG. 1 is a schematic diagram of the apparatus used to measure the rateof absorption of water into a concentrated lithium bromide-watersolution for determination of half pressure times (HPT).

FIG. 2 is a graphical illustration of the pressure history of therefrigerant water vapor as comparatively absorbed by a working solutionwith and without an additive of this invention.

A suitable absorbent for a refrigeration system of the type describedherein is a solution of lithium bromide and water. The concentration ofthe lithium bromide in the strong solution leaving the generator may beabout 65 percent, and the lithium bromide concentration may range from55 to 65 weight percent in the system. A suitable refrigerant is water.Our additives are added to the system to increase heat transfer in thecondenser and absorber and consequently improve the performance incapacity of the refrigeration system. Although the mechanism of theobserved phenomena is not entirely understood, and we do not wish to bebound by theory, some experimental evidence suggests the possibilitythat a condition for an effective increase in the capacity of anabsorption machine is the existence of the additives in the region ofthe absorber and the condenser tube.

We have discovered that certain additives substantially improve theoverall capacity of a conventional refrigeration machine. The workingfluid of this invention comprises water as a refrigerant in combinationwith lithium bromide as the absorbent to which is added 4-heptanol,decyl alcohol, 2- phenoxyethanol, nonanol, 3-methyl-2-heptanone, methyloctanoate, 3-methyl-2-heptanol, l-heptanol, l-octanol, 2-hexanol,2-octanol, 2-methyl-2-hexanol, 2-methyl-2-heptanol, 6- methyl-2heptanoland Z-heptanol. These additives are supplied to the lithiumbromide-water fluid in the range of 50 to 1000 parts per million on aweight basis of the lithium bromide-water solution. It has been foundthat these additives produce a substantial increase in the overallcapacity of the refrigeration machine by improving the heat transfer inthe absorber, by promoting dropwise condensation of the refrigerantvapor on the exterior surfaces of the condenser tubes, and by improvingpurging of relatively noncondensible gases from the absorber section ofthe machine. Of the above additives, more preferred groups are2-octanol, 2-methyl-2-hexanol, 2- methyl-Z-heptanol,6-methyl-2-heptanol, 2-hexanol, 2-heptanol, and mixtures thereof. Asdisclosed in more detail below, the most preferred additive isZ-heptanol, which may be present in amounts ranging from 50 to 1000parts per million, but is most conveniently added in an amount of about400 parts per million.

In order to ascertain the effectiveness of such additives, water vaporabsorption data was obtained and compared for the case when no additiveis present in the lithium bromide water solution. This data can bepresented in many forms, but a convenient and relevant method is toanalyze and express this data as the half pressure time (HPT) inseconds. The HPT is defined as the time required for the water vaporpressure in an enclosed volume (flask F in FIG. 1) to reach one-half ofits initial value as a result of water vapor absorption by lithiumbromide solution in a separate volume connected therewith (flask D inFIG. 1).

The method of ascertaining the l-IPTs is as follows, the descriptionreferring particularly to FIG. I. A salt solution is introduced into thejacketed flask, D, and water is introduced into holder E. After all ofthe air is pumped out of the solution of flask D and from the water offlask E, valve C connecting the system to the vacuum pump is closed. Thedesired amount of additive is then introduced into the flask D in theamounts previously discussed. With valve B isolating flask D from flaskE, water from flask E is allowed to vaporize into water vapor holder Fthrough valve A and then valve A is closed. The pres sure of the watervapor of holder F is measured by manometer U whereas the pressure of thelithium bromide solution in flask D is measured by pressure gauge Pwhile the solution in flask D is thermostated.

To start the experiment, valve B is suddenly opened to allow quickcommunication of vapor holder F with the vapor phase of flask D. As soonas valve B is opened, an instantaneous increase of the pressure abovethe solution in flask D is experienced. As water vapor is continuouslyabsorbed by the solution in flask D, the pressure decreases untilfinally it reaches a minimum value equivalent to the vapor pressure ofthe solution in flask D at the temperature of the thermostated jacket.During all this time, the pressure is recorded as a continuous functionof time. A typical example of such pressure time curves is shown in FIG.2. In this chart, the pressure-time relationship from time zeroincreasing to the opening of valve B is shown for a number of cases suchas the case when no absorption is taking place, the case when thesolution of lithium bromide and water has no additive, and when theadditive 2- heptanol is added to the solution of lithium bromide-watersolution.

F IG. 2 illustrates the benefit of additives, specifically Z-heptanolthe preferred additive of this invention, to lithium bromide-waterworking solution. The pressure in flask D of FIG. 2 is plotted as afunction of time for various conditions (solutions) in flask D, whichcontains the working solutions. The solid line A represents the initialvapor pressure of the lithium bromide-water solution in flask D. Thefinal LiBr-H O solution vapor pressure in flask D is substantiallyidentical and is therefore represented by the horizontal dashed line A.In the first case, where there is no absorption of flask F water vaporby flask D solution, the pressure history in flask D is represented byline B. Thus, line B represents the vapor pres sure of the water inflask D, which remains constant, as in the cases where no, or onlywater, refrigerant is present in flask D, or an absorbent in therefrigerant in flask D does not absorb vapor from F. Line C representsthe condition where the absorbent in the water refrigerant in flask D isLiBr. The water vapor pressure in flask D falls along the curve C. LineD shows the effect of the additive Z-heptanol present in the LiBr-H Oworking solution in flask D. The water vapor from flask F is rapidlyabsorbed by the solution in flask D in accordance with the curve D. Theeffect is immediate and rapid as seen from the fact that the water vaporpressure peaks at D well below both the no absorption and no additivelevels.

The following nonlimiting examples are illustrative of the invention.

EXAMPLE 1 The HPTs of the various additives of this invention weremeasured according to the procedure given above and compared to the casewhen no additive is present in the lithium bromide-water solution. Thetypical solution is taken as 56 weight percent lithium bromide in waterand the half pressure times of the various additives added to the 56weight percent lithium bromide solution in amounts varying from through600 parts per million were ascertained. The results obtained for theadditives are presented in table 1.

TABLE 1 Comparison of Additives (56 wt. percent LiBr in H O Solution)Amount of Additive. p.p.m (By Weight) ln addition to these tests, and toverify the direct relationship existing between the HPT data and theperformance in an actual absorber, a number of field tests wereconducted utilizing two nominal 5-ton refrigeration capacity Arkla-waterchillers, Models R 60-96. Such Arkla-water chillers are commerciallyavailable.

Unit No. l was operated with a lithium bromide-water solution for aperiod of time and under the test conditions its capacity was recorded.Then 200 parts per million of 6- methyl-Z-heptanol were added to unit 1and the unit was run for some time and its capacity was recorded.Finally, in the same unit 200 parts per million of the additiveZ-heptanol were added and after a period of operation its capacity wasrecorded.

Unit 2 was operated for a period with no additive present in its lithiumbromide-water working fluid and its capacity was recorded. Then 200parts per million of the additive Z-heptanol were added and the newcapacity was recorded. ln all of these cases an increase in capacity ofbetween 13.4 and 31.7 percent was observed over the capacity of thesemachines operating without an additive. The results of these tests areshown in detail on table 2.

TABLE 2.FIELD TEST RESULTS Capacity gain Amount of Chilled Chilled Unitover no additive, water, water, capacity, additive, Arkla-water lp.p.rn. gaL/min, AT F. Btu/hr. percent Unit N 0. 1:

Case I No Additive 12. 0 8. 1 48, 500 t). 0 Case II 6methy1-2-heptanoladditive 200 12. 0 9. 2 55, 000 13. 4 Case III Mixture of 6-methylQ-heptanol and 2-heptano1 additive 400 12. O 9. 8 58, 500 20. 6Unit No. 2:

Case I No Additive l1. 9 7.8 46, 300 0. 0 Case II Z-Heptanol additive200 12. 5 9. 8 61, 000 13. 7

1 Chiller nominal 5 ton capacity Model B 60-96.

In view of the teachings set forth herein, it can be seen that theadditives significantly improve the capacity of absorption refrigerationmachines of the type using lithium bromidewatcr as the working fluidwhen added to such working fluids. While we have described our inventionin connection with specific embodiments thereof, they are to beunderstood as being merely illustrative and not by way of limitation ofthe scope of our invention, which is defined solely in the appendedclaims which should be construed as broadly as the prior art willpermit.

We claim:

I. A working fluid for absorption refrigeration systems comprising wateras a refrigerant, and an aqueous solution of lithiurn bromide as anabsorbent, to which is added an additive selected from 4-heptanol,2-phenoxyethanol, 3-methyl-2-heptanone, methyl octanoate,3-methyl-2-heptanol, 2-hexanol, 2-

octanol, 2-methyl-2-hexanol, 2-methyl-2-heptanol, 6-methyl- Z-heptanol,Z-heptanol, and mixtures thereof. and said additives are present in saidworking fluid in amounts ranging from about 50 to 1000 parts per millionby weight.

2. A working fluid as in claim I wherein said additives are 2- octanol.2-methyl-2-hexanol. Z-methyI-Z-heptanol, 6-mcthyl- Z-heptanol.2-heptanol, and mixtures thereof.

3. A working solution as in claim I wherein said additive is Z-heptanol.

4. A working solution as in claim 3 wherein said 2-heptanol is presentin said aqueous solution of lithium bromide in a range of 50-l000 p.p.m.by weight.

5. A working solution as in claim 4 wherein said Z-heptanol is presentin said aqueous solution of lithium bromide in an amount ofabout 400p.p.m. by weight.

2. A working fluid as in claim 1 wherein said additives are 2-octanol,2-methyl-2-hexanol, 2-methyl-2-heptanol, 6-methyl-2-heptanol,2-heptanol, and mixtures thereof.
 3. A working solution as in claim 1wherein said additive is 2-heptanol.
 4. A working solution as in claim 3wherein said 2-heptanol is present in said aqueous solution of lithiumbromide in a range of 50-1000 p.p.m. by weight.
 5. A working solution asin claim 4 wherein said 2-heptanol is present in said aqueous solutionof lithium bromide in an amount of about 400 p.p.m. by weight.